1. Field of the Invention
The present invention relates to a friction device for controlling force, and a friction damper comprising such a device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
There are already known friction devices for controlling force, namely used in friction damping devices. The general principle consists of using two friction surfaces with relative displacement pressed against each other so as to transform the normal force at said surfaces into a friction force capable of creating a damping force.
Generally, these damping devices comprise at least two organs, one of which is movable in displacement with respect to the other one under the action of an external force. The displacement is likely to cause the relative displacement of two friction surfaces subjected to pressing forces, so as to transform said external force into a friction force and to thus dampen the displacement of said movable organ.
The friction devices are used in different fields, both, non-restrictively, in that of air, road and railway transport for damping, breaking, clutch or transmission, torque-limiting systems and in that of weapons for damping the return of the firearms.
However, controlling the force of a friction device occurs through the normal pressing force, so that it directly depends on the friction coefficient of the materials of the friction surfaces, which leads to uncertainties about the returned force or about its repeatability and stability over the time.
From GB 2 60 740 is also known a shock absorber for vehicles aimed at being used to damper springs. The invention comprises friction elements such as discs, movable in displacement with respect to each other under the action of a displacement, associated with springs the action and the thrust of which are controlled to put the friction elements under pressure. This pressure varies according to the direction of said movement, i.e. this pressure is applied in one direction, while it is suppressed in the other direction. To this end, one portion of the friction elements is screwed onto a threaded coaxial element, and the thrust of the springs is axially applied on the side opposite the one including the other portion of the friction elements. Thus, the pivoting in the direction of the screwing conjugated with the thrust causes the friction to increase, while the pivoting in the other direction tends to separate the friction elements and to suppress the contact between them.
The operation of such a shock absorber is of the “on-off” type and does not permit control of the force. It is provided for assisting a damper system, i.e. it oscillates according to the normal operation of the damper system and is blocked when the oscillation reaches a determined magnitude in the direction of compression of the spring of said damper system. It does therefore not permit calibration of the damping force in only one direction, since, in order the remove the blocking, a change of direction of the force is necessary.